Transcript electric current

Electric Charge
7.1
Positive and Negative Charge
• Atoms contain particles called protons,
neutrons, and electrons.
• Protons and
electrons have
electric charge,
and neutrons have
no electric charge.
Electric Charge
7.1
Positive and Negative Charge
• Protons have positive electric charge and
electrons have negative electric charge.
• The amount of positive charge on a proton
equals the amount of negative charge on an
electron.
Electric Charge
7.1
Positive and Negative Charge
• An atom contains equal numbers of protons
and electrons, so the positive and negative
charges cancel out and an atom has no net
electric charge.
• Objects with no net charge are said to be
electrically neutral.
Electric Charge
7.1
Transferring Charge
• Compared to the electrons in carpet atoms,
electrons are bound more tightly to the
atoms in the soles of your shoes.
• When you walk on
the carpet, electrons
are transferred from
the carpet to the
soles of your shoes.
Electric Charge
7.1
Transferring Charge
• The soles of your shoes have an excess of
electrons and become negatively charged.
• The carpet has lost electrons and has an
excess of positive charge.
• The accumulation
of excess electric
charge on an
object is called
static electricity.
Electric Charge
7.1
Conservation of Charge
• According to the law of conservation
of charge, charge can be transferred
from object to object, but it cannot be
created or destroyed.
• Whenever an object becomes charged,
electric charges have moved from one
place to another.
Electric Charge
7.1
Charges Exert Forces
• Unlike charges attract each other, and like
charges repel each other.
• The force between
electric charges
also depends on the
Opposite charges attract
distance between
charges. The force
Like charges repel
decreases as the
charges get farther
apart.
Electric Charge
7.1
Charges Exert Forces
• The force between any two objects that are
electrically charged decreases as the objects
get farther apart.
• This force also depends on the amount of
charge on each object.
• As the amount of charge on either object
increases, the electrical force also increases.
Electric Charge
7.1
Conductors and Insulators
• If you reach for a metal doorknob after
walking across a carpet, you might see a
spark.
• The spark is
caused by
electrons
moving from
your hand to the
doorknob.
Electric Charge
7.1
Conductors
• A material in which electrons are able to
move easily is a conductor.
• The best electrical conductors are metals.
• The atoms in metals have electrons that are
able to move easily through the material.
Electric Charge
7.1
Insulators
• A material in which electrons are not able
to move easily is an insulator.
• Electrons are held tightly to atoms in
insulators.
• Most plastics are insulators.
• The plastic coating
around electric wires
prevents a dangerous
electric shock when
you touch the wire.
Electric Charge
7.1
Charging Objects
• Rubbing two materials together can result
in a transfer of electrons.
• Then one material is left with a positive
charge and the other with an equal amount
of negative charge.
• The process of transferring charge by
touching or rubbing is called charging
by contact.
Electric Charge
7.1
Charging at a Distance
• Because electrical forces act at a distance,
charged objects brought near a neutral object
will cause electrons to rearrange their
positions on the neutral object.
Electric Charge
7.1
Charging at a Distance
• The balloon on the left is neutral. The
balloon on the right is negatively charged.
It produces a positively charged area on
the sleeve by repelling electrons.
• The rearrangement of
electrons on a neutral
object caused by a
nearby charged object
is called charging by
induction.
Electric Charge
7.1
Lightning
• Lightning is a large static discharge.
• A static discharge is a transfer of charge
between two objects because of a buildup
of static electricity.
• A thundercloud is a mighty generator of static
electricity. As air masses move and swirl in
the cloud, areas of positive and negative
charge build up.
Electric Charge
7.1
Lightning
• Eventually, enough charge builds up to cause
a static discharge between the cloud and the
ground.
• As the electric charges
move through the air,
they collide with atoms
and molecules. These
collisions cause the
atoms and molecules
in air to emit light.
Electric Charge
7.1
Thunder
• Lightning also generates powerful sound
waves.
• The electrical energy in a lightning bolt rips
electrons off atoms in the atmosphere and
produces great amounts of heat.
• The heat causes air in the bolt's path to
expand rapidly, producing sound waves that
you hear as thunder.
Electric Charge
7.1
Grounding
• A discharge can occur any time that charge
builds up in one area.
• Providing a path for charge to reach Earth
prevents any charge from building up.
• Earth is a large, neutral object that is also a
conductor of charge.
Electric Charge
7.1
Grounding
• Any object connected to Earth by a good
conductor will transfer any excess electric
charge to Earth.
• Connecting an
object to Earth with
a conductor is called
grounding.
Electric Charge
7.1
Detecting Electric Charge
• The presence of electric charges can be
detected by an electroscope.
• One kind of electroscope is made of two thin,
metal leaves attached to a metal rod with a
knob at the top.
• The leaves are allowed to hang freely from
the metal rod.
Electric Charge
7.1
Detecting Electric Charge
• When the device is not charged, the leaves
hang straight down.
• Notice the position
of the leaves on
the electroscope
when they are A
uncharged, B
negatively
charged, and C
positively charged.
Section Check
7.1
Question 1
The law of conservation of charge states that
___________.
Section Check
7.1
Answer
Charge can be transferred from object to
object, but it cannot be created or destroyed.
Section Check
7.1
Question 2
Which of the following is the best conductor
of electricity?
A. copper
B. rubber
C. wood
D. water
Section Check
7.1
Answer
The answer is A. The best electrical conductors
are metals.
Section Check
7.1
Question 3
The accumulation of excess electric charge
on an object is called __________.
A. lightning
B. static electricity
C. static discharge
D. thunder
Section Check
7.1
Answer
The answer is B. Lightning is a large static
discharge that emits light.
Electric Current
7.2
Current and Voltage Difference
• The net movement of electric charges in a
single direction is an electric current.
• In a metal wire, or any material, electrons are
in constant motion in all directions. As a
result, there is no net movement of electrons
in one direction.
Electric Current
7.2
Current and Voltage Difference
• When an electric current flows in the wire,
electrons continue their random movement,
but they also drift in the direction that the
current flows.
• Electric current is measured in amperes.
Electric Current
7.2
Voltage Difference
• In some ways, the electric force that causes
charges to flow is similar to the force acting
on the water in a pipe.
• Water flows from higher pressure to lower
pressure.
Electric Current
7.2
Voltage Difference
• In a similar way, electric charge flows from
higher voltage to lower voltage.
• A voltage difference
is related to the force
that causes electric
charges to flow.
Voltage difference is
measured in volts.
Electric Current
7.2
Electric Circuits
• This figure shows an electric current doing
work by lighting a lightbulb.
• A closed path that electric current follows is a
circuit.
• If the circuit is
broken by removing
the battery, or the
lightbulb, or one of
the wires, current
will not flow.
Electric Current
7.2
Batteries
• To keep an electric current continually
flowing in the electric circuit a voltage
difference needs to be maintained in the
circuit.
• A battery can provide the voltage difference
that is needed to keep current flowing in a
circuit.
• Current flows as long as there is a closed path
that connects one battery terminal to the other
battery terminal.
Electric Current
7.2
Dry-Cell Batteries
• A cell consists of two electrodes surrounded
by a material called an electrolyte.
• The electrolyte enables charges to move from
one electrode to the other.
Electric Current
7.2
Dry-Cell Batteries
• One electrode is the carbon rod, and the other
is the zinc container.
• The electrolyte is a moist paste containing
several chemicals.
• The cell is called a
dry cell because the
electrolyte is a moist
paste, and not a
liquid solution.
Electric Current
7.2
Wet-Cell Batteries
• A wet cell contains two connected plates
made of different metals or metallic
compounds in a conducting solution.
• A wet-cell battery
contains several
wet cells connected
together.
Electric Current
7.2
Lead-Acid Batteries
• Most car batteries are lead-acid batteries.
• A lead-acid battery contains a series of six
wet cells made up of lead and lead dioxide
plates in a sulfuric acid solution.
• The chemical reaction in each cell provides a
voltage difference of about 2 V, giving a total
voltage difference of 12 V.
Electric Current
7.2
Resistance
• As the electrons flow through the filament in
a lightbulb, they bump into the metal atoms
that make up the filament.
• In these collisions,
some of the
electrical energy of
the electrons is
converted into
thermal energy.
Electric Current
7.2
Resistance
• Eventually, the metal filament becomes hot
enough to glow, producing radiant energy
that can light up a dark room.
Electric Current
7.2
Resisting the Flow of Current
• Resistance is the tendency for a material to
oppose the flow of electrons, changing
electrical energy into thermal energy and
light.
• With the exception of some substances that
become superconductors at low temperatures,
all materials have some electrical resistance.
• Resistance is measured in ohms ().
Electric Current
7.2
Temperature, Length, and
Thickness
• The electric resistance of most materials
usually increases as the temperature of the
material increases.
• The resistance of an object such as a wire
also depends on the length and diameter of
the wire.
Electric Current
7.2
Temperature, Length, and
Thickness
• The resistance of a wire, or any conductor,
increases as the wire becomes longer.
• The resistance also increases as the wire
becomes thinner.
Electric Current
7.2
The Current in a Simple Circuit
• A simple electric circuit contains a source of
voltage difference, such as a battery, a device,
such as a lightbulb, that has resistance, and
conductors that connect the device to the
battery terminals.
• When the wires are connected to the battery
terminals, current flows in the closed path.
Electric Current
7.2
The Current in a Simple Circuit
• The voltage difference, current, and
resistance in a circuit are related.
• If the voltage difference doesn't change,
decreasing the resistance increases the
current in the circuit.
• If the resistance doesn't change, increasing
the voltage difference increases the current.
Electric Current
7.2
Ohm's Law
• According to Ohm's law, the current in a
circuit equals the voltage difference divided
by the resistance.
• If I stands for the electric current, Ohm's law
can be written as the following equation.
Electric Current
7.2
Ohm's Law
• Ohm's law provides a way to measure the
resistance of objects and materials. First the
equation above is written as:
• An object is connected to a source of voltage
difference and the current flowing in the
circuit is measured.
• The object's resistance then equals the
voltage difference divided by the measured
current.
Section Check
7.2
Question 1
__________ is the net movement of electric
charges in a single direction.
A. An open circuit
B. Electric current
C. Proton flow
D. Voltage
Section Check
7.2
Answer
The answer is B. Electric forces in a material
cause electric current to flow.
Section Check
7.2
Question 2
What is a volt?
Answer
A volt is the unit of
measurement for voltage
difference, which is related
to the force that causes
electric charges to flow.
Section Check
7.2
Question 3
What is the tendency for a material to
oppose the flow of electrons called?
Answer
The tendency for a material to oppose the
flow of electrons is called resistance, and is
measured in ohms.
Electrical Energy
7.3
Series and Parallel Circuits
• Circuits usually include three components.
One is a source of voltage difference that can
be provided by a battery or an electrical
outlet.
• Another is one or more devices that use
electrical energy.
• Circuits also include conductors such as
wires that connect the devices to the source
of voltage difference to form a closed path.
Electrical Energy
7.3
Series Circuits
• One kind of circuit is called a series circuit.
• In a series circuit,
the current has only
one loop to flow
through.
• Series circuits are
used in flashlights
and some holiday
lights.
Electrical Energy
7.3
Open Circuit
• How can one faulty bulb cause a whole string
of lights to go out?
• When any part of a series circuit is
disconnected, no current flows through the
circuit.
• This is called an open circuit. The burnedout bulb causes an open circuit in the string
of lights.
Electrical Energy
7.3
Parallel Circuits
• Houses are wired with parallel circuits.
• Parallel circuits contain two or more
branches for current to move through.
• The current can
flow through
both or either of
the branches.
Electrical Energy
7.3
Parallel Circuits
• Parallel circuits have several advantages.
• When one branch
of the circuit is
opened, such as
when you turn a
light off, the
current continues
to flow through the
other branches.
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Electrical Energy
7.3
Household Circuits
• The wiring in a house must allow for the
individual use of various appliances and
fixtures.
• This wiring is mostly a combination of
parallel circuits connected in an organized
and logical network.
Electrical Energy
7.3
Household Circuits
• The main switch and circuit breaker or fuse
box serve as an electrical headquarters for
your home.
Electrical Energy
7.3
Household Circuits
• Parallel circuits branch out from the breaker
or fuse box to wall sockets, major appliances,
and lights.
Electrical Energy
7.3
Household Circuits
• To protect against overheating of the wires,
all household circuits contain either a fuse or
a circuit breaker.
Electrical Energy
7.3
Fuses
• An electrical fuse contains a small piece of
metal that melts if the current becomes too
high.
• When it melts, it causes a break in the circuit,
stopping the flow of current through the
overloaded circuit.
Electrical Energy
7.3
Fuses
• To enable current to flow again in the circuit,
you must replace the blown fuse with a new
one.
• Too many appliances in use at the same time
is the most likely cause for the overheating of
the circuit.
Electrical Energy
7.3
Circuit Breaker
• A circuit breaker contains a piece of metal
that bends when the current in it is so large
that it gets hot.
• The bending causes a switch
to flip and open the circuit,
stopping the flow of current.
• Circuit breakers usually can
be reset by pushing the
switch to its "on" position.
Electrical Energy
7.3
Electric Power
• The reason that electricity is so useful is that
electrical energy is converted easily to other
types of energy.
• The rate at which electrical energy is
converted to another form of energy is the
electric power.
Electrical Energy
7.3
Calculating Electric Power
• Electric power can be calculated from the
following equation.
• The unit for power is the watt (W). Because
the watt is a small unit of power, electric
power is often expressed in kilowatts (kW).
• One kilowatt equals 1,000 watts.
Electrical Energy
7.3
Electrical Energy
• Electric companies charge by the amount
of electrical energy used, rather than by the
electric power used.
• Electrical energy usually is measured in units
of kilowatt hours (kWh) and can be
calculated from this equation:
Electrical Energy
7.3
The Cost of Using Electrical
Energy
• The cost of using the appliance can be
computed by multiplying the electrical
energy used by the amount the power
company charges for each kWh.
• For example, if a 100-W lightbulb is left on
for 5 h, the amount of electrical energy used
is
Electrical Energy
7.3
The Cost of Using Electrical
Energy
• The cost of
using some
household
appliances is
given in this
table, where the
cost per kWh is
assumed to be
$0.09/kWh.
Section Check
7.3
Question 1
What is an open circuit?
Answer
An open circuit is a series circuit in which
one part is disconnected. This prevents
current from flowing through the circuit.
Section Check
7.3
Question 2
The rate at which electrical energy is
converted to another form of energy is called
__________.
A. electrical fuse
B. electrical switching
C. electric power
D. thermal energy
Section Check
7.3
Answer
The answer is C. Electric power is the rate
of energy conversion and is measured in
watts.
Section Check
7.3
Question 3
Which of the following equations is used to
calculate electric power?
A.
B.
C.
D.
P = IV
P = I/V
P = V/I
P = AV
Section Check
7.3
Answer
The answer is A. Electric power is equal to
the current, I, multiplied by the voltage
difference, V.
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